Cavity resonator magnetron



Aug. 11, 1953 o. A. LARDELL! CAVITY RESONATOR MAGNETRON Filed March 31, 1949 3 m 2 5 u M Patented Aug. 11, 1953 CAVITY RESONATOR MAGNETRON Otto Albert Lardelli, Baden, Switzerland, assignor to Patelhold Patentverwertungs- & Elektro- Holding A.-G., Glarus, Switzerland Application March 31, 1949, Serial No. 84,570 In Switzerland December 17, 1948 4 Claims.

This invention relates to cavity resonator ma netrons, and more particularly to novel resonator cavity constructions which improve the stability of oscillation at natural resonant frequency of the cavity resonator.

The cavity resonator is of conducting material and in the form of a toroid of preferably rectangular cross-section, as viewed in radial section, with the inner cylindrical wall divided into a plurality of anode segments which are connected, in alternation, to the opposite annular side walls of the cavity resonator. A large surface cathode or the equivalent is arranged axially within the cylindrical space defined by the anode segments, and a unidirectional magnetic field is established within and axially of that cylindrical space.

A magnetron of this kind exhibits especially great frequency stability when it oscillates at the fundamental or natural resonant frequency of the cavity resonator. With this oscillation mode, it is well known that the anode segments secured alternately to the two side surfaces provide the oscillation capacity of the cavity, and that the oscillatory currents circulate only in planes passing through the magnetron axis inside the cavity. A very simple magnetic field is then set up in the cavity resonator with the lines of force perpendicular to the afore-mentioned planes. With this extremely simple oscillation mode, the anode segments must be short in relation to the wave length and they then constitute true capacity electrodes of the cavity and, in the direction of the axis, show practically no diiference in alternating potential.

An object of the present invention is to provide a magnetron of the cavity resonator type which precludes the generation of currents of other than the natural resonant frequency of the cavity resonator. a magnetron of the type stated in which the cavity resonator is so constructed as to suppress parasitic currents which tend to develop along the respective annular side walls between anode segments connected to those walls. An object is to provide magnetrons of the type stated which include couplings between anode seg-l ments to maintain the same alternating current potential upon all anode segments which are connected to the same side wall of the cavity resonator. More specifically, an object is to provide a magnetron of the cavity resonator type in which one or both annular side walls of the cavity resonator are slotted radially to prevent current flow circumferentially of the annular side walls.

An object is to provide These and other objects and the advantages of the invention will be apparent from the following specification when taken with the accompanying drawings in which:

Fig, 1 is a substantially central section through an all-metal magnetron embodying the invention;

Fig. 2 is a transverse section on line 22 of Fig. 1; and

Fig. 3 is a perspective view, with parts broken away, of a magnetron in which the cavity resonator is enclosed within an evacuated glass envelope.

In the drawings, the reference characters S1 and S2 identify the interleaved anode segments which define the inner cylindrical surface of a cavity resonator having side walls i, 2 and an outer cylindrical wall 3. A large surface cylindrical cathode 4 is located axially within the cylindrical space defined by the anode segments, and it is screened at its ends from anode potential by disk-shaped plates 5, 6, respectively. Lead-in wires 7, 8 for the cathode or cathode heater extend through bores in the two-part permanent magnet 9 which establishes a magnetic field axially of the cathode 4 and the cylindrical space between the anode segments S1, S2. The current developed at the anode segments oscillates back and forth over the inner surface of the cavity resonator to produce a magnetic field which issues from the plane of the drawing, Fig. l, at one side of the magnetron axis and enters again at the other side. In the constructional form shown, the energy of the magnetron is coupled out inductively by a coupling loop H1 extending into this field. This loop, insulated by the glass lead-through seal ll, extends to the exterior of the magnetron for energizing a load circuit. It is important to understand that the high frequency magnetic field, due to its annular shape surrounding the cathode, induces an inductive voltage in the cathode. To prevent this voltage from sending undesired stray currents to the exterior of the magnetron through the cathode leads, a blocking impedance nonresonant at the generated frequency is provided adjacent one of the cathode leads, for example the lead 1. The blocking impedance may be, as illustrated, a coaxial assembly comprising an outer tube [2, constituting a part of the all-metal envelope of the magnetron, and an inner tube I3 through which the lead-in wire 1 extends axially. The outer ends of tubes I2 and [3 are conductively connected, and the length of this coaxial assembly is equal to one-fourth of the wave-length of the generated oscillations. With this or equivalent blocking arrangements, a leakage of high frequency energy along the cathode leads is efiectively prevented.

The cathode leads 1, 8 are supported on, and insulated from the all-metal housing by insulating bushings l4, 15 respectively. Tubes 16, I1 through which cooling water is circulated are soldered or welded to the side plates I, 2, respectively, near the points of connection of the anode segments to those plates.

It has been noted that the energy output of cavity resonator magnetrons constructed as above described can not be increased to an order which, with regard to thermally permissible limits, is theoretically possible. This saturation or cut-off efiect has been observed with magnetrons having ten or twelve anode segments and is more striking with a greater number of anode segments, for example fifty or more, which are desirable for high power. I have found that this limitation upon the efiective power output is due to a shift or jumping over to modes of operation differing from the simple mode of oscillation at the fundamental resonant frequency of the cavity resonator. In these undesired modes, the high frequency magnetic field is broken up into separate fractions, for example into two halves. All anode segments connected to one side wall of the cavity resonator do not have the same alternating current potential under such conditions and, for example, one half of each group of anode segments S1 or S2 connected to the same side wall I or 2, respectively, may be at a positive potential while the other half is at a negative potential. This undesired polarity condition results when the magnetic field set up by the oscillatory current inadvertently breaks into two halves. When this occurs, high frequency oscillatory currents flow along the side walls I and 2 between the momentarily positive and negative anode segments of each group S1 and S2.

According to the invention, the frequency of oscillation is stabilized by preventing the development of circulating currents in or along the side walls of the cavity resonator. One method of suppressing the circulating wall currents, and the disturbance of the high frequency field which they cause, is to connect all of the anode segments of one group by a metallic ring l8, see Figs. 1 and 2. This short-circuiting ring is located at substantially the central transverse plane of the cavity resonator and, as shown, connects all of the S2 segments. The manner in which the parasitic circulating currents are suppressed will be apparent from a consideration of two symmetrically located field lines [9 of the distorted magnetic field, see Fig. 2. With respect to these field lines, the ring I8 and segments S2 form short circuits in which eddy currents which would be induced by this distorted field substantially completely compensate for it. The field distortion is thus suppressed between the centrally located ring 18 and the side wall 2, i. e. in the lower half of the cavity resonator as viewed in Fig. 1, and it will be apparent that, by reason of symmetry, distorted fields can not occur in the top half if they are suppressed in the lower half. By suppressing the distorted magnetic fields, the parasitic currents which they would induce in the side walls between anode segments of the same group are of course suppressed.

As shown in Fig. 3, the cavity resonator may be supported within an evacuated glass envelope 2i by fins or metal plates 22, 23 which are soldered or welded to the wall 3 of the cavity resonator and to wires or lead-ins extending through the pressed glass stem 24 of the envelope. Shield plates 5', 6' with flared extensions are welded to and supported by the leadin wires 25, 26 of the cathode heating circuit and, in turn, they support a large diameter cathode which, as illustrated, has the form of spiral filament 21, the diameter of the spiral being appropriately selected according to the number of anode segments and the anode diameter as de scribed in the copending application of Fritz Liidi, Serial No. 628,528, filed Nov. 15, 1945, which matured into Patent No. 2,597,506 on May 20, 1952.

The circulating currents along the side walls are suppressed in a very simple manner by providing radial slots in one or both of the annular walls of the cavity resonator. As shown, the side wall 2' is provided with two radial slots 28, 29 which are spaced circumferentially by about The annular side wall is thus slit radially and preferably, as shown, from the inside outwardly substantially to its outer edge.

A consideration of the dual magnetic fields, as indicated by flux lines I9 of Fig. 2, set up by the parasitic oscillations indicates that it would not be possible to suppress the parasitic oscillations by slitting a side wall in only one diametrical plane. With such a construction, the parasitic oscillation could be set up with the undesired wall currents at a maximum on a diameter perpendicular to the diametrical plane of the radial slits. Good stability is attained with two slits spaced circumferentially by about one-third of the circumference, and the slits may be cut in the same side wall or one slit may be in one side wall and the second slit in the other side wall. Both side walls may be provided with two or more radial slits and, as a limiting case, both side walls may be provided with slits between each pair of adjacent anode segments. In this construction, the sides of the cavity resonator are formed by radial extensions of the several anode segments.

The invention is not limited to the embodiments herein shown and described as other constructions which may occur to those familiar with the design and construction of cavity resonator magnetrons fall within the spirit and scope of the invention as set forth in the following claims.

I claim:

1. In a magnetron, a cavity resonator of conducting material and in toroidal form having a pair of annular side walls cooperating with 00- axial inner and outer cylindrical walls, the outer Wall being circumferentially complete and the inner cylindrical wall being defined by two sets of interleaved anode segments, there being the same number of more than four anode segments in each set and adjacent anode segments being connected in alternation to opposite annular side walls of the cavity resonator, a cathode within the cylindrical space defined by said anode segments, and means for stabilizing oscillation of the magnetron at the natural resonant frequency of the cavity resonator; said stabilizing means preventing current fiow tangentially between the segments on the said annular side walls of the cavity resonator and comprising slits extending radially outwardly from said inner cylindrical Wall substantially to the outer cylindrical wall and located in different diametrical planes through the axis of said cylindrical walls.

2. In a magnetron, the invention as recited in 5 claim 1, wherein two of said radial slits are spaced circumferentially by substantially 120.

3. In a magnetron, the invention as recited in claim 2, wherein both slits of said pair are in the same side wall of the cavity resonator.

4. In a magnetron, the invention as recited in claim 2, wherein said two radial slits constitute said stabilizing means.

OTTO ALBERT LARDELLI.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Number Number Name Date Gutton et a1. Feb. 14, 1939 Burns Dec. 9, 1947 Spencer Dec. 16, 1947 Spencer Sept. 28, 1948 Nordsieck Mar. 1949 Pierce Mar. 14, 1950 Crawford et a1. Apr. 25, 1950 FOREIGN PATENTS Country Date France Sept. 8, 1941 

